This invention relates to development apparatus for using toner to develop electrostatic images carried on an insulating surface, and more particularly, to apparatus for reducing contamination of such surface by toner dust.
In electrography, it is common to form an electrostatic image on an insulating surface of an electrophotographic member in the form of a drum or web and to develop that image by applying toner particles thereto. In many commercial applications, the toner is either transferred in an image-wise configuration to another surface and then fixed or is fixed to the insulating surface itself. In processes in which the toner is transferred from the insulating surface prior to fixing, the insulating surface generally is reused.
Triboelectric developing systems are frequently used in the development of electrostatic images. In such systems, finely divided toner particles are held to the surface of much larger carrier particles by electrostatic charges created by triboelectrification, forming a mixture (herein called a developer). When the developer is brought into contact with an electrostatic image, the charge on the image attracts the triboelectrically charged toner and overcomes the attraction of the carrier for the toner thereby developing the image.
Among triboelectric developing systems, the most commonly used are cascade systems and magnetic brush systems. In cascade systems, gravity is used to roll developer across the image. Because cascade systems use gravity as their primary moving force, they are necessarily speed limited. In automatic machines, a cascade recirculation system generally requires substantial machine space.
In magnetic brush systems, the carrier particles are ferromagnetic in nature. These ferromagnetic carrier particles are held to an applicator surface, for example, a nonmagnetic cylinder, in a bristle formation, by magnets located inside the cylinder. The bristles are brushed across a surface carrying an electrostatic image. The electrostatic attraction between the toner and the charged image overcomes the triboelectrically created attraction between toner and ferromagnetic particles and the image is developed. Areas of the image exerting less attractive force on the toner than is exerted by the carrier are cleaned of toner as they are brushed.
In one form of magnetic brush development apparatus, measured amounts of toner are added in a sump to the developer to maintain a proper developer mixture of toner, and carrier (magnetic) particles. The toner is folded into the developer mixture by feed and return augers and a mixing wheel. The feed auger moves the developer from the sump through a passageway into a development station wherein the magnetic brush(s) is disposed. The return auger moves developer from the station back through a passageway into the sump. This type of development apparatus will be described more fully in the detailed description which follows. The toner added to the sump is in the form of a very fine toner dust. In the sump a dust cloud of toner which is not strongly attracted to the carrier in the developer may be formed above the developer surface. The augers will cause circulation of air from the sump into the development station. Thus toner dust can enter the development station and deposit on the electrophotographic member. Also, toner dust can leak out of the development station into the copier machine and cause contamination of vital machine components.